In this article we review Raman studies of defects and dopants in graphene as well as the importance of both for device applications. First a brief overview of Raman spectroscopy of graphene is presented. In the following section we discuss the Raman characterization of three defect types: point defects, edges, and grain boundaries. The next section reviews the dependence of the Raman spectrum on dopants and highlights several common doping techniques. In the final section, several device applications are discussed which exploit doping and defects in graphene. Generally defects degrade the figures of merit for devices, such as carrier mobility and conductivity, whereas doping provides a means to tune the carrier concentration in graphene thereby enabling the engineering of novel material systems. Accurately measuring both the defect density and doping is critical and Raman spectroscopy provides a powerful tool to accomplish this task.

https://iopscience.iop.org/article/10.1088/0953-8984/27/8/083002/pdf

Raman characterization of defects and dopants in graphene

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-020259.pdf

Gate-tunable phase transitions in thin flakes of 1T-TaS$_{2}$

Infrared spectroscopy is a powerful tool widely used in research and industry for a label-free and unambiguous identification of molecular species. Inconveniently, its application to spectroscopic analysis of minute amounts of materials, for example, in sensing applications, is hampered by the low infrared absorption cross-sections. Surface-enhanced infrared spectroscopy using resonant metal nanoantennas, or short “resonant SEIRA”, overcomes this limitation. Resonantly excited, such metal nanostructures feature collective oscillations of electrons (plasmons), providing huge electromagnetic fields on the nanometer scale. Infrared vibrations of molecules located in these fields are enhanced by orders of magnitude enabling a spectroscopic characterization with unprecedented sensitivity. In this Review, we introduce the concept of resonant SEIRA and discuss the underlying physics, particularly, the resonant coupling between molecular and antenna excitations as well as the spatial extent of the enhancement and...

Surface-Enhanced Infrared Spectroscopy Using Resonant Nanoantennas.

We calculate the double resonant (DR) Raman spectrum of graphene, and determine the lines associated to both phonon-defect processes, and two-phonons ones. Phonon and electronic dispersions reproduce calculations based on density functional theory corrected with GW. Electron-light, -phonon, and -defect scattering matrix elements and the electronic linewidth are explicitly calculated. Defect-induced processes are simulated by considering different kind of idealized defects. For an excitation energy of $\epsilon_L=2.4$ eV, the agreement with measurements is very good and calculations reproduce: the relative intensities among phonon-defect or among two-phonon lines; the measured small widths of the D, $D'$, 2D and $2D'$ lines; the line shapes; the presence of small intensity lines in the 1800, 2000 cm$^{-1}$ range. We determine how the spectra depend on the excitation energy, on the light polarization, on the electronic linewidth, on the kind of defects and on their concentration. According to the present findings, the intensity ratio between the $2D'$ and 2D lines can be used to determine experimentally the electronic linewidth. The intensity ratio between the $D$ and $D'$ lines depends on the kind of model defect, suggesting that this ratio could possibly be used to identify the kind of defects present in actual samples. Charged impurities outside the graphene plane provide an almost undetectable contribution to the Raman signal.

/pdf/theory-of-double-resonant-raman-spectra-in-graphene-55hadiikn1.pdf

Theory of double-resonant Raman spectra in graphene: intensity and line shape of defect-induced and two-phonon bands

Terahertz time-domain spectroscopy is used to measure the complex conductivity of nanometer-thick gold films evaporated on silicon substrates in the far-infrared spectral region from $0.2\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}2.7\phantom{\rule{0.3em}{0ex}}\mathrm{THz}$. With increasing film thickness a characteristic crossover from an insulating to a conducting state via a percolation transition is observed. Of particular interest is the characteristic non-Drude behavior close to the transition. Whereas effective medium theory is inconsistent with our measurements in this regime, the Drude-Smith model, a generalization of the classical Drude model which incorporates carrier localization through backscattering, provides excellent fits to the observed complex conductivity. Applying this model we observe extreme values for the carrier scattering time at the percolation threshold.

Terahertz conductivity of thin gold films at the metal-insulator percolation transition

Electrons confined in ultrathin metal films provide a window on the peculiar world of quantum mechanics.

Quantum size effects in metallic nanostructures

The resistivity of ultrathin single-crystalline Pb and Pb-In layers with thicknesses d smaller than the bulk mean free path l, is measured during deposition onto Si(111)-(6\ifmmode\times\else\texttimes\fi{}6)Au surfaces at about 110 K. The structure of the layers is monitored by reflection high-energy electron diffraction (RHEED). Oscillations of the RHEED specular beam intensity are highly correlated with fine structures of the resistivity. The quantum-size-effect theory is used for a quantitative analysis of the data. The fine structure, volume impurities, small-scale roughness, and large-scale thickness fluctuations are taken into account. The impact of the layer-by-layer growth mode of ultrathin metal films on the thickness dependence of the resistivity is discussed.

Experimental evidence for quantum-size-effect fine structures in the resistivity of ultrathin Pb and Pb-In films

The Hall coefficient ${R}_{H}$ of ultrathin epitaxial Pb films is determined experimentally. A comparison with electrical conductivity data leads to the conclusion that the investigated Pb layers behave like a size-quantized metal. Pronounced variations of ${R}_{H}$ with the film thickness were found. The observed reversal of the sign of ${R}_{H}$ is discussed within the available theory of the quantum size effect describing the galvanomagnetic properties of metals. We find that the observed phenomenon cannot be explained by the free electron model of a quantized layer.

Quantized Hall effect in ultrathin metallic films.

The quantum-size-effect structure of ultrathin Pb and Pb-In alloy films on Si(111) surfaces is studied by photoemission spectroscopy in the thickness range from 0 to about 30 monolayers and analyzed, taking into account electron scattering and film structure, which is characterized by high-energy-reflection-electron diffraction

Mieczysław Jałochowski

Papers

Quantum size effects in metallic nanostructures

Experimental evidence for quantum-size-effect fine structures in the resistivity of ultrathin Pb and Pb-In films

Quantized Hall effect in ultrathin metallic films.

Photoemission from ultrathin metallic films: Quantum size effect, electron scattering, and film structure.

Gold-induced ordering on vicinal Si(111)